Pulverizing mills are used to pulverize coal, limestone and other solid materials. In the case of coal, gravel sized coal enters the mill and is pulverized into a powder. The powder is carried out of the pulverizer by a high velocity air stream and into a furnace where it explosively burns to heat steam which drives a turbine to generate electricity. The pulverizers are designed to operate continuously, except during periods of repair. Examples of these kinds of coal pulverizers are in U.S. Pat. No. 4,705,223 by Dibowski et al; U.S. Pat. No. 4,694,994 by Henne et al; U.S. Pat. No. 4,679,739 by Hashimoto et al; U.S. Pat. No. 4,522,343 by Williams; U.S. Pat. No. 4,491,280 by Bacharach; and U.S. Pat. No. 4,717,082 by Guido et al.
The pulverizing is accomplished by directing the coal onto grinding tables which interface with pulverizing rollers. The rollers are each mounted on a separate roller assembly shaft, and each roller assembly shaft is mounted on a clamshell door in the pulverizer. Typically, the grinding table is a disk-shaped member with an annular groove or raised circumferential edge in the top surface. The grinding table rotates so that the annular groove mates with the rollers. The coal is introduced from the top of the assembly and feeds by gravity to the annular groove where it is pulverized as the grinding table rotates under the rollers. The rollers and grinding table are massive; each roller weighs several tons and is on the order of five feet in diameter.
The pulverizer may use a rotating grinding table with stationary roller assemblies, as described in U.S. Pat. No. 4,717,082 by Guido et al (the contents of which are hereby incorporated by reference) or, alternatively, may use a stationary grinding table and several rotating roller assemblies. The roller assemblies may also be independently biased against the grinding table so that vibration and shock on one roller will not be transferred to all the other rollers, as described in the Guido patent.
The roller assemblies typically include a rigid hard steel "tire" or "tread" portion mounted on a rigid softer hub of WCB steel. The assembled tire and hub have roughly the configuration of an automobile tire and hub but, of course, are much larger and are rigid. The roller assemblies are exposed to extreme conditions. They typically revolve at 200 to 300 revolutions per minute and operate at a temperature around 600.degree. to 700.degree. F. The mill occasionally catches fire. The fire is smothered with steam and is then cooled, resulting in large and fast temperature changes in the rollers.
The two-piece roller assembly comprising a tire and hub is the source of a number of costly problems in the pulverizer. A principal problem is that the tires wear out. The wear rate varies depending on the hardness of the coal and the amount of time that the pulverizer is not operating, but in general it is not unusual for the tire to wear out in less than a year. When the tire is worn out, the roller assembly must be removed from the pulverizer, the tire must be removed from the hub, the replacement tire must be mounted on the hub, and the rebuilt assembly must be replaced into the pulverizer. This requires a great deal of labor. In addition, it takes a long time, and the pulverizer can not operate during that time. The down time is typically a week or two, at a cost of many thousands of dollars per day. Electric utilities pass that cost on to rate payers.
Repairing the existing roller assemblies presents other difficulties. The existing roller assemblies require costly replacement tires. The fit between the massive tire and hub is usually quite poor and becomes worse as the assembly wears, especially since the tire is relatively hard and the hub is relatively soft. This results in vibration, abrasion and shock as the rollers crush the coal and it ultimately destroys the soft hub as well as the hard tire. As the assembly wears, the fineness of the coal deteriorates and the energy required to turn the rollers increases, resulting in a loss of mill efficiency. When a fire is smothered with steam, the differential shrinkage and expansion between the tire and hub causes extreme stress and even cracking of the tire or hub. None of the existing devices described in the patents cited above or elsewhere adequately addresses these problems.